1. The decision to down bot turret or not is a lot more complicated than you make it seem. There are many factors to consider. Bottom line is don't ruin a good thing. For example the enemy bot is vayne and sona and you are killing vayne and cutting off CS. Do you really want to down the turret? How are the other lanes holding up? etc etc
2. Regarding supports carrying. Again that gets complicated. The bigger the skill gap between you and everyone on the enemy team the more being support is a bad idea. However in my opinion unless its a very large gap I wouldn't be the range adc either because like you stated the range adc also relies on the support. If its a very large skill gap then I would do the range adc.
If someone say ended up at 800 elo and they went from there to say 1500 doing support then thats very impressive but unless they are simply not capable of playing any other role as good it was not a wise decision. You mentioned picking blitz alistar and sona but blitz and alistar are going to be banned...at the very least blitz will be banned because if any support is going to carry its that one.
Say Ego Ignaxio and Xpecial some how magically end up in a low elo game on opposite teams on smurf accounts (yes I specified that so xpecial doesnt ban kennen). Ego is kennen mid and Xpecial is support (and yes blitz is banned) what team do you think is going to win? I would put money on Ego infact I don't think it would be even close.
Edit: Oh and don't kid yourself support is not easy. I mean from a gaming skill stand point like if ez is hard because of skill shots then sure yeah support is easy. However as far as tactical knowledge goes...support is not easy and requires a good understanding of what your champions and enemy champions are capable of.
Edit2: of course some supports are significantly easier than others
Everytime someone messes up, ima copy and paste the link to this post for them.
Ima try to be as polite as i can, say hello all, lets have a good game, and make a great team comp, in every lobby i go to.
Your completly right, i usually try to fill in whatever role is needed by picking my champion last. But sometimes people just dont want to pick and wait til lthe 10 second marker. :\
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